Empirical models of pressure and density in Saturn's interior: Implications for the helium concentration, its depth dependence, and Saturn's precession rate     

Ravit Helled  Gerald Schubert 《Icarus》2009,199(2):368-377

We present ‘empirical’ models (pressure vs. density) of Saturn's interior constrained by the gravitational coefficients J₂, J₄, and J₆ for different assumed rotation rates of the planet. The empirical pressure-density profile is interpreted in terms of a hydrogen and helium physical equation of state to deduce the hydrogen to helium ratio in Saturn and to constrain the depth dependence of helium and heavy element abundances. The planet's internal structure (pressure vs. density) and composition are found to be insensitive to the assumed rotation rate for periods between 10h:32m:35s and 10h:41m:35s. We find that helium is depleted in the upper envelope, while in the high pressure region (P?1 Mbar) either the helium abundance or the concentration of heavier elements is significantly enhanced. Taking the ratio of hydrogen to helium in Saturn to be solar, we find that the maximum mass of heavy elements in Saturn's interior ranges from ∼6 to 20 M_⊕. The empirical models of Saturn's interior yield a moment of inertia factor varying from 0.22271 to 0.22599 for rotation periods between 10h:32m:35s and 10h:41m:35s, respectively. A long-term precession rate of about ^0.754″ yr⁻¹ is found to be consistent with the derived moment of inertia values and assumed rotation rates over the entire range of investigated rotation rates. This suggests that the long-term precession period of Saturn is somewhat shorter than the generally assumed value of 1.77×¹⁰⁶ years inferred from modeling and observations.  相似文献   

X-ray interferometry with transmissive beam combiners for ultra-high angular resolution astronomy     

Gerald K. Skinner  John F. Krizmanic 《Experimental Astronomy》2009,27(1-2):61-76

Interferometry provides one of the possible routes to ultra-high angular resolution for X-ray and gamma-ray astronomy. Sub-micro-arc-second angular resolution, necessary to achieve objectives such as imaging the regions around the event horizon of a super-massive black hole at the center of an active galaxy, can be achieved if beams from parts of the incoming wavefront separated by 100s of meters can be stably and accurately brought together at small angles. One way of achieving this is by using grazing incidence mirrors. We here investigate an alternative approach in which the beams are recombined by optical elements working in transmission. It is shown that the use of diffractive elements is a particularly attractive option. We report experimental results from a simple 2-beam interferometer using a low-cost commercially available profiled film as the diffractive elements. A rotationally symmetric filled (or mostly filled) aperture variant of such an interferometer, equivalent to an X-ray axicon, is shown to offer a much wider bandpass than either a Phase Fresnel Lens (PFL) or a PFL with a refractive lens in an achromatic pair. Simulations of an example system are presented.  相似文献   

Interannual variations and future change of wintertime extratropical cyclone activity over North America in CCSM3     

Haiyan Teng  Warren M. Washington  Gerald A. Meehl 《Climate Dynamics》2008,30(7-8):673-686

Climatology and interannual variations of wintertime extratropical cyclone frequency in CCSM3 twentieth century simulation are compared with the NCEP/NCAR reanalysis during 1950–1999. CCSM3 can simulate the storm tracks reasonably well, although the model produces slightly less cyclones at the beginning of the Pacific and Atlantic storm tracks and weaker poleward deflection over the Pacific. As in the reanalysis, frequency of cyclones stronger than 980 hPa shows significant correlation with the Pacific/North America (PNA) teleconnection pattern over the Pacific region and with the North Atlantic Oscillation (NAO) in the Atlantic sector. Composite maps are constructed for opposite phases of El Nino-Southern Oscillation (ENSO) and the NAO and all anomalous patterns coincide with observed. One CCSM3 twenty-first century A1B scenario realization indicates there is significant increase in the extratropical cyclone frequency on the US west coast and decrease in Alaska. Meanwhile, cyclone frequency increases from the Great Lakes region to Quebec and decreases over the US east coast, suggesting a possible northward shift of the Atlantic storm tracks under the warmer climate. The cyclone frequency anomalies are closely linked to changes in seasonal mean states of the upper-troposphere zonal wind and baroclinicity in the lower troposphere. Due to lack of 6-hourly outputs, we cannot apply the cyclone-tracking algorithm to the other eight CCSM3 realizations. Based on the linkage between the mean state change and the cyclone frequency anomalies, it is likely a common feature among the other ensemble members that cyclone activity is reduced on the East Coast and in Alaska as a result of global warming.  相似文献   

Spline models of contemporary, 2030, 2060 and 2090 climates for Mexico and their use in understanding climate-change impacts on the vegetation     

Cuauhtémoc Sáenz-Romero  Gerald E. Rehfeldt  Nicholas L. Crookston  Pierre Duval  Rémi St-Amant  Jean Beaulieu  Bryce A. Richardson 《Climatic change》2010,102(3-4):595-623

Spatial climate models were developed for México and its periphery (southern USA, Cuba, Belize and Guatemala) for monthly normals (1961–1990) of average, maximum and minimum temperature and precipitation using thin plate smoothing splines of ANUSPLIN software on ca. 3,800 observations. The fit of the model was generally good: the signal was considerably less than one-half of the number of observations, and reasonable standard errors for the surfaces would be less than 1°C for temperature and 10–15% for precipitation. Monthly normals were updated for three time periods according to three General Circulation Models and three emission scenarios. On average, mean annual temperature would increase 1.5°C by year 2030, 2.3°C by year 2060 and 3.7°C by year 2090; annual precipitation would decrease ?6.7% by year 2030, ?9.0% by year 2060 and ?18.2% by year 2090. By converting monthly means into a series of variables relevant to biology (e. g., degree-days > 5°C, aridity index), the models are directly suited for inferring plant–climate relationships and, therefore, in assessing impact of and developing programs for accommodating global warming. Programs are outlined for (a) assisting migration of four commercially important species of pine distributed in altitudinal sequence in Michoacán State (b) developing conservation programs in the floristically diverse Tehuacán Valley, and (c) perpetuating Pinus chiapensis, a threatened endemic. Climate surfaces, point or gridded climatic estimates and maps are available at http://forest.moscowfsl.wsu.edu/climate/.  相似文献   

Process identification through rejection of model structures in a mid‐mountainous rural catchment: observations of rainfall–runoff response,geophysical conditions and model inter‐comparison     

Thomas Graeff  Erwin Zehe  Dominik Reusser  Erika Lück  Boris Schröder  Gerald Wenk  Hermann John  Axel Bronstert 《水文研究》2009,23(5):702-718

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Constraining slip rates and spacings for active normal faults   总被引：1，自引：0，他引：1  

Patience A. Cowie  Gerald P. Roberts 《Journal of Structural Geology》2001,23(12)

Numerous observations of extensional provinces indicate that neighbouring faults commonly slip at different rates and, moreover, may be active over different time intervals. These published observations include variations in slip rate measured along-strike of a fault array or fault zone, as well as significant across-strike differences in the timing and rates of movement on faults that have a similar orientation with respect to the regional stress field. Here we review published examples from the western USA, the North Sea, and central Greece, and present new data from the Italian Apennines that support the idea that such variations are systematic and thus to some extent predictable. The basis for the prediction is that: (1) the way in which a fault grows is fundamentally controlled by the ratio of maximum displacement to length, and (2) the regional strain rate must remain approximately constant through time. We show how data on fault lengths and displacements can be used to model the observed patterns of long-term slip rate where measured values are sparse. Specifically, we estimate the magnitude of spatial variation in slip rate along-strike and relate it to the across-strike spacing between active faults.  相似文献   

The annual march of the topoclimatic spatial patterns of net radiation in southern California     

Prof. Werner H. Terjung  Ronald N. Kickert  Ronald J. Kochevar  Jack P. Mrowka  Simon O. Ojo  Gerald L. Potter  Stanton E. Tuller 《Theoretical and Applied Climatology》1969,17(1):21-50

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Effects of salinity on the condition and survival of zebra mussels (Dereissena polymorpha)     

Bruce W. Kilgour  Gerald L. Mackie  Mark A. Baker  Roger Keppel 《Estuaries and Coasts》1994,17(2):385-393

The objectives of this study were to determine the tolerance of various life stages of zebra mussels to salinity; determine the extent to which acclimation events in estuarine systems, affect tolerance of zebra mussels; and determine the effects of salinity on health or condition of adult zebra mussels. At high temperatures (18–20°C), the condition of zebra mussels is reduced at salinities above 1%.. However, at lower temperatures (3–12°C), the optimum salinity for zebra mussels is 2–4%.. The incipient lethal salinity of post-veligers is near 2%., of larger adults (5–15 mm) between 2%. and 4%., and of veliger larvae near 4.5%.. Zebra mussels are able to acclimate to slowly changing salinities (i.e., 1%. d^?1) such that the time to 50% mortality of a population should be greater than 1 yr at temperatures near 20°C and salinites up to 8%.  相似文献   

Evolution of pore microstructures during healing of grain boundaries in synthetic calcite rocks     

David L. Olgaard  John D. Fitz Gerald 《Contributions to Mineralogy and Petrology》1993,115(2):138-154

The morphologies of calcite grain boundaries were analyzed to provide insight into the evolution of pore networks in unfractured rock. Two synthetic calcite rocks were fabricated by hot isostatically pressing (HIP-ing) dried analytical-grade powders of pure CaCO₃ and CaCO₃ plus 5% Al₂O₃ at 600° C and 200 MPa confining pressure for 3 hours (HIP-1). Some samples were HIPed a second time at different temperatures and pressures to investigate the stability of the structures (HIP-2a-c). SEM and TEM were used to image both grain faces and grain boundary cross-sections. Structures on grain faces vary from open shallow basins with peripheral rims, to labyrinths of irregular ridges and channels, to isolated circular depressions. All of these structures are mirrored across the plane between grain faces. The grain size in both the single and two-phase samples increased markedly during HIP-1. Migrating boundaries either dragged pores along or broke away leaving grain interiors dotted with small voids. The structures present after HIP-1 were not stable but evolved considerably in a way dependent on the conditions of the HIP-2. Confining pressure had the most pronounced effect. With low confining pressure, the grain-boundary porosity evolved into isolated circular depressions but the total pore volume did not noticeably decrease. With high confining pressure, the pore volume virtually disappeared. The structures present after HIP-1 are strikingly similar to those that develop in intragranular cracks during healing. We infer that grain boundaries and intragranular cracks heal by similar processes. Decomposition, localized melting, impurities, and anisotropic surface energies played no evident role in forming the grain-boundary structures. The timing of the formation of the porosity and of the subsequent healing processes is more difficult to ascertain. Some structures appear to have evolved gradually throughout the constant, high temperature stage of HIPing. The most obvious structures, however, appear to have evolved on grain boundary cracks that opened during cooling.  相似文献   

Stability of Rayleigh-Benard convection rolls and bimodal flow at moderate Prandtl number     

John A. Whitehead Jr.  Gerald L. Chan   《Dynamics of Atmospheres and Oceans》1976,1(1):33-49

Experimental observations of cellular convection between two rigid, horizontal, conducting boundaries are reported for two different cases. First, the stability of two-dimensional roll convection of various wavenumbers and Rayleigh numbers is investigated in fluids of Prandtl numbers 16 and 2.7. The results qualitatively agree with earlier observations by Busse and Whitehead of fluid with Prandtl number 126 but they differ somewhat quantitatively. Second, the stability of the bimodal flow, consisting of two rolls of differing and perpendicular wavenumber, is observed to be stable for given bandwidths and ranges of Rayleigh numbers when the configuration of the bimodal flow, consisting of two sets of rolls at right angles, is flawless — without any disruptions in the periodic matrix. The stability range differs from earlier experiments and from our own experiments in which the bimodal planform is uncontrolled.  相似文献